In previously filed copending application Ser. No. 426,323 filed Dec. 19, 1973, it was proposed to measure the smoke density in the smoke pipe or flue of a furnace, by means of radiation from a uniformly radiating source such as an incandescent light bulb. The radiation falls on a photosensitive cell, preferably a cadmium-sulphide type cell, whose resistance to electrical flow varies with the intensity of the light impinging upon it. As the smoke interferes with the passage of light through the smoke pipe, and therefore regulates the quantity of light impinging upon the photocell, the resistance of the cell varies in inverse proportion to the quantity of smoke passing through the smoke pipe. However, as the level of smoke to be measured and maintained is very low in the pertinent range, a smoke gathering device or amplifier is used to concentrate the smoke level in the area of the light beam.
By applying a fixed voltage across the resistance of a divider network including the photocell, a varying voltage is created. This voltage is amplified and fed to a solid state analog-to-digital converter. This solid state converter creates, for example, a multi-bit word which represents the level of smoke detected by the photocell. These words trigger an appropriate triac switching circuit for controlling the resistance coupled to a modulating electric motor, in turn, controlling its position, thereby controlling the air-oil ratio of the furance and, finally, the smoke level in the smoke pipe.
The electrical circuit described in my copending application operates satisfactorily under most conditions. However, there are certain conditions in which improvement is possible. Thus, for example, the circuit of my copending application did not respond directly to different kinds of fuel which might be employed and, therefore, necessitated manual adjustment of the mechanical controls. Moreover, the original circuit described in my prior copending application operated in a manner which tended to be incremental whereas I have found that there are certain applications in which a more continuous type of operation is preferred.